It is well known to use electromagnetic radiation to detect the presence of objects (e.g. handheld detectors used for detecting objects on or under the ground, and walk-through arches at airports).
However, the conventional detectors used at airports may be unable to determine the dimensions of objects to any significant degree, and thus may be unable to distinguish between objects of different types, i.e. harmless (belt buckles, cameras), and potentially dangerous (guns, knives).
The detection of concealed weapons, especially handguns, may be a very great problem for security applications that currently cannot be policed without a non-portable system, for example random checks in an urban environment. The use of microwaves (electromagnetic waves with wavelengths in the centimeter to millimeter range) may provide a means for the standoff detection and identification of concealed conducting items such as handguns and knives. Large metal objects, such as handguns, may give a significantly different and generally larger response when irradiated by low power microwaves than that from the human body, clothing and/or benign normally-carried objects. The larger response may be detected using a combination of antenna and sensitive receiver.
By actively illuminating an object with wide-range swept and/or stepped frequency microwave and/or millimeter wave radiation, the frequency response of the return signal may give the range and/or information regarding dimensions of the object. This method may be substantially equivalent to using a fast microwave pulse and measuring the response as function of time, as used in conventional RADAR. Selecting a part of the return signal within a particular range may aid the positive identification of the suspect object and may also help to reject background signals. The analysis of the time response may give further information as to the dimensions of the target. This technique may also be applied to the detection of dielectric layers, such as, for example, an explosive vest strapped to a suicide bomber (see Active millimeter wave detection of concealed layers of dielectric material, Bowring N. J., Baker J. G., Rezgui N., Southgate M., Proceedings of the SPIE 6540-52 2007; and A sensor for the detection and measurement of thin dielectric layers using reflection of frequency scanned millimetric waves, Bowring N. J., Baker J. G., Rezgui N., Alder J. F. Meas. Sci. Technol. 19 024004 (7 pp) 2008). However, such techniques have not been heretofore used for detecting and measuring metal objects.
A system based on swept frequency RADAR has been proposed (U.S. Pat. Nos. 6,359,582 and 6,856,271 and published application US2007/0052576). In the disclosed systems, the frequency may be swept by typically by 1 GHz around about 6 GHz. The depth resolution that is achievable is therefore only 15 cm, thus the system may not give details of the objects. The detection relies on comparing gross features of the signal as a whole with similar suspicious and benign signals to which the system had been previously exposed. Also the measurement of polarization properties of the scattered signal may be used.
In the aforementioned patent documents, the low frequency of operation makes the angular resolution of the antennae poor and the wide field of view makes it difficult to single out particular targets and/or to determine on which part of the target the threat is situated. This may be improved by changing to higher frequencies where microwave optics becomes effective. This may be particularly important for explosives detection where the contrast from the body signal is low. Systems working at higher frequencies but still with a limited bandwidth have been proposed by Gorman et al (U.S. Pat. No. 6,967,612) and by Millitech (U.S. Pat. No. 5,227,800). Many systems have been produced to enable images of the target to be obtained using either active microwave illumination or the passive thermal emission of the target (SPIE 2007). These systems use multi-detector arrays and some form of mechanical scanning. Passive systems, though giving more realistic images, tend to be slow and show poor contrast for dielectric targets. Active illumination systems can be acquired faster, but may suffer from strong reflections from benign objects such as the human body, which make it difficult to distinguish from metal threat objects. All scanning systems may require complex human or Artificial Intelligence interaction to interpret the image and/or to pick out the suspect features. This makes their deployment in many applications difficult.
It is apparent that systems which can identify threat objects at stand-off may have many applications, where conventional metal detector booths are inappropriate. These may include covert surveillance and mobile operation in streets and buildings. Portable, compact and cost-effective systems are not presently available and this invention seeks to address this need.